An ion source for a mass spectrometer comprises: a capillary having a nozzle for emitting a nebulized fluid sample; an electrode of the capillary; a high voltage power supply electrically coupled to the electrode; a second electrode disposed within or configurable to be disposed within a path of the nebulized fluid sample, wherein the capillary and capillary electrode are configurable so as to ionize the nebulized fluid sample by electrospray ionization and the second electrode is configurable so as to ionize the nebulized sample by atmospheric pressure chemical ionization and wherein the second electrode is moveable between positions such that the second electrode is and is not disposed within the path of the nebulized fluid sample, respectively.
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